The reasons I still use Amiga OS:
boots in seconds.
Can add/remove programs by commenting out a line of text
(unlike registry)
Can make a new bootable partition just by dragging and dropping what's on another partition.
You can easily learn exactly what every file and folder does,
they are not all jammed together with weird names like windows.
e.g: Monitor drivers go in the monitor folder, printer drivers in the printers folders, libraries in libs etc. Just move em into storage if you don't want to use them. (...which is called storage!)
Very low overheads: it's there to run other programs, and to be efficent and easy to learn. it doesn't need a gig of ram and a 1Ghz processor just to run itself.
Excelent Multi tasking.
Very responsive.